The uranium isotopic composition of modern stromatolites forming in Shark Bay, Western Australia

Stromatolites represent some of the earliest evidence for life and are valuable geochemical archives for understanding the rise of oxygen on early Earth. Metal redox proxies in carbonates, such as stable uranium isotopes (²³⁸U/²³⁵U), are useful for assessing the oxidation state of ancient waterbodies, but may also be sensitive to local water chemistry and early sedimentary diagenesis. This requires the validation of such proxies in modern environments before applying them to ancient carbonates. Here we measure the U isotopic composition of modern stromatolites forming in Hamelin Pool in Shark Bay, Western Australia – a large hypersaline marine embayment and the largest modern example of stromatolite development globally. Actively-growing stromatolite tops from Shark Bay exhibited a narrow range of δ²³⁸U from -0.30 to -0.33‰, corresponding to an offset of ca. +0.1‰ from seawater. Such an offset has not been found in other biotic marine carbonates, which exhibit seawater-like δ²³⁸U (ca. -0.4‰), but is consistent with findings from carbonate co-precipitation experiments. One hypothesis for our measured +0.1‰ offset is the elevated Ca concentration of the hypersaline Shark Bay seawater relative to open seawater. This results in a greater proportion of dissolved U present as Ca₂UO₂(CO₃)₃, which is expected to be isotopically lighter than other U species and not incorporated during carbonate mineral formation. Higher δ²³⁸U up to +0.11‰ were measured in the deeper stromatolite laminae, consistent with the expected U isotope signatures for U reduction. Stromatolite radiocarbon ages show that the diagenetic modification of U occurs within ~1 ka and may be considered syndepositional on geological timescales. These results from the deeper stromatolite laminae support the application of a ca. -0.4‰ correction factor to the δ²³⁸U of stromatolites formed in oxic waterbodies, similar to other biotic carbonates. It is unclear whether the additional +0.1‰ offset found in stromatolite tops is particular to seawater chemistry of Shark Bay or a general feature of microbial carbonate precipitation. This warrants investigation of the δ²³⁸U proxy in other modern environments where stromatolites proliferate.